Fender Frontman 212 troubleshooting buzz

[walter]

I have a Fender Frontman 212 that has a constant buzz in the output when it is on and nothing plugged in. It’s not D.C. at the speaker, so I assumed it would be an easy fix, bad filter caps. I scoped the supply rails and found ripple on the negative supply. I replaced the filter caps, but the hum remained. It appears to have some fluid intrusion, so everything is suspect. I jumpered a filter cap across the negative rail to ground as close to the output as I could find and the hum remained.
I located a schematic and took some measurements. I then plugged a guitar into the input and the hum stopped. I also tried plugging into the power amp in and that stopped the hum too. The input jacks have a switch to the MUTE circuit, aha!
I connected an o-scope to the mute circuit between Q7 and R72 and the hum stopped. I took some screen shots and verified the switching worked. I noticed the scope has a three prong plug. I tried my older scope with a ground lift and the hum returned. With the grounded o-scope, the voltages are fine, with the ungrounded scope, or with no scope attached the negative rail is down a little from -40.1v to -37.5v when nothing is plugged into the input.
The hum seems to be related to the mute circuit and may be ground related. I’m not sure what to try next, I’ll ohm out the grounds. If nothing else, I could disable the mute circuit, but it has a thermal switch RT2 that could protect the amp from meltdown, so it may be the last fix for this thing. Any suggestions on what to try next are welcome.
This amp belongs to a friend, he asked me if it was dirty. I didn’t think much of it at the time, but it does have some greasy paste like substance on the faceplate. I suspect this amp was rescued from his house after the runoff from a chemical fire flooded the place. Fluid intrusion is usually not good and corrossion can continue after the first repair, so It may be a lost cause.
Here is a link to an article about the fire, he lived in the house two doors down, the water flushed the chemicals filling up his basement: https://www.kiro7.com/news/two-alarm-fire-burning-fremont/82183450

 

[walter]

Power supply

 

[walter]

MUTE circuit

 

[walter]

Schematic, full.

 

[PRR]

Start disassembling for distilled water rinsedown and hot-air dry.

Compare to the $170 value of a used Frontman.

Prepare paperwork: this cost/loss should be covered by insurance. The customer's or maybe a general claim against the chemical company. "8hrs cleaning @ $25/hr, vs $159-$199 going prices".

 

[walter]

I gave up and disconnected the MUTE line from the jacks, the thermal switch remains. I snipped W38 that goes to R73 and C46. My buddy sold his house, but this amp wasn't from the house. It was from a secret underground rehearsal space known as the Bunker. There is a lid in the floor of the room that leads to a storm drain. There is a constant funk and mildew there. The amp was orphaned, and he acquired it when his drummer joined a new band and kicked them out of his space. The amp works now, I told him to sell it to buy drugs.

 

[mjrippe]

The amp works now, I told him to sell it to buy drugs.

Good advice for some of the cheap modern "Fenders".

 

[CJ]

first thing to do with those amps is replace all the J111's. deoxit jacks and pots and plug it in and look for the smoke.  be sure to check the back of the amp for free swag, usually all i get is a set list full of originals that nobody will ever hear.

we also have a bunker of doom here in Oly WA . it's called the flop House.  people go in there for a show and they never come out. get's hot down there, word of mouth party pad, a little slice of life you want to forget,

 

[JMFahey]

first thing to do with those amps is replace all the J111's.

What for?

 

[Tubetec]

Ive had a few Fender 212's for repair
Fender used different jacks over the years ,often I found jacks that were meant to make ground to the chassis with spikes or a retained toothed washer where it mounts ,maybe with the chemicals you have a reaction between where the steel on the jack sockets and the aluminum of the chassis meet . I think Ive also seen a situation on that amp where bad pot grounding to chassis occured and caused odd things to happen.

Probably best thing for it Walt ,back out the door as fast as you can .